Blood Pressure

What two terms can express blood supply to a tissue?

Flow and perfusion

Perfusion

Distribution of blood into the capillaries so the tissues can be supplied

Flow

The movement of blood through a vessel, tissue, or organ

What is total flow like at rest?

Quite constant, remains equal to the cardiac output (typically 5.25 L/min)

Hemodynamics

Physical principle of blood flow based on pressure and resistance

Hemodynamics formula

F is proportional to ΔP/R (F= flow, ΔP = difference in pressure, R = resistance)

What is flows relationship to pressure?

Directly proportional. As pressure rises, flow rises, as pressure sinks, flow sinks

What is flows relationship to resistance?

Inverse relationship. As resistance increases, flow decreases, as flow increases, resistance decreases

Blood pressure

The force that blood exerts against a vessel wall

What determines blood pressure?

Cardiac output, blood volume, and resistance to flow

Which organ primarily regulates blood volume?

Kidneys

What are some factors that can increase BP?

• Blood volume increasing

• Heart rate increasing

• Stroke volume increasing

• Blood viscosity increasing

• Peripheral resistance increasing

Where is BP measured?

Brachial artery of arm using sphygmomanometer

Systolic pressure

BP taken during ventricular contraction

Diastolic pressure

BP taken during ventricular relaxation

Normal BP value

120/80 mmHg (systolic/diastolic)

Pulse pressure

Difference between systolic and diastolic pressure (systolic - diastolic)

Mean arterial pressure (MAP)

The mean pressure one would obtain by taking measurements at several intervals throughout the cardiac cycle. Normal is 70-100 mmHg, determined by diastolic pressure + (one-third of pulse pressure)

Hypertension

High blood pressure. 140/90 at rest

Hypotension

Below 90/60 at rest, caused by blood loss, dehydration, and anemia

What happens to BP as we age?

Tends to rise due to factors such as arteriosclerosis and atherosclerosis

Arteriosclerosis

Stiffening of arteries due to deterioration of elastic tissues of artery walls

Atherosclerosis

Build up of lipid deposits in vessels that become plaques

Blood pressure decreases the further it gets from which heart structure?

The left ventricle

Blood volume

Sum of volumes of plasma and formed elements. Varies with age, body size, and gender but is typically 5 L for adults (4-5 for females, 5-6 for males) and accounts for 8% of body weight.

What is blood volumes relationship to BP?

Directly proportional. Any factor that changes blood volume can change BP.

Peripheral Resistance

Force of friction between blood and walls of blood vessels

PR’s relation to BP

BP must overcome PR in order to flow. Factors that change PR also change BP.

As PR increases, BP does what?

Increases

Three variables that PR hinges on

Blood viscosity, vessel length, and vessel radius

Viscosity

Thickness of blood, which forms the difficulty with which molecules of fluid flow past each other. The greater the viscosity, the greater the resistance to blood flow, which causes a higher BP. Blood cells and plasma proteins increase viscosity.

Vessel Length

The farther liquid travels through a tube, the more cumulative friction it encounters. Pressure and flow decline with distance.

Vessel Radius

Most powerful influence over flow and is the only significant way of controlling resistance. Vasoreflexes change the vessel radius.

Vasoconstriciton

When smooth muscle of tunica media contracts, decreasing vessel radius

Vasodilation

Relaxation of the smooth muscle, allowing blood pressure to expand vessel

Three reasons why blood velocity decreases from the aorta to the capillaries

• Greater distance, more friction to reduce speed

• Smaller radii of arterioles and capillaries offers more resistance

• Farther from heart, the number of vessels and their total cross-sectional area become greater and greater

What happens to blood velocity as it moves from the capillaries back to the vena cava?

It increases due to veins creating less resistance than capillaries because they are larger.

Why does the blood in veins never regain the velocity it had when it was in larger arteries?

Veins are further from the heart and are more compliant than arteries

What are the most significant point of control over PR and flow, producing half of the total PR?

Arterioles

BP is the product of what?

CO x PR

Three factors that control BP

Cardiac output, peripheral resistance, and baroreceptor reflexes

Sympathetic baroreceptor reflexes

Increase HR, contractility, vasoconstriction, and BP

Parasympathetic baroreceptor reflexes

Decrease HR, which decreases BP

Vasoreflexes

Quick and powerful ways of altering blood pressure and flow

Three ways of controlling vasomotor activity

Local control, neural control, and hormonal control

Autoregulation (local control)

The ability of tissues to regulate their own supply

Vasoactive chemicals (local control)

Substances secreted by platelets, endothelial cells, and perivascular tissue to stimulate vasomotor responses

Reactive hyperemia (local control)

If blood supply is cut off and then restored, flow increases above normal

Angiogenesis (local control)

Growth of new blood vessels

How does the nervous system exert control over BP?

By influencing blood vessel size. The vasomotor center of the medulla exerts sympathetic control over blood vessels throughout the body, stimulating most vessels to constrict but dilating vessels in cardiac muscle to meet demands of exercise

Baroreceptor reflexes

Automatic, negative feedback response to change in blood pressure. Baroreceptors in the carotid sinuses detect increases in BP, causing the glossopharyngeal nerve to send signals to the brain stem.

Baroreflexes govern what form of regulation of BP?

Short-term

Chemoreflex

An automatic response to changes in blood chemistry, especially pH and concentrations of O2 and CO2

Chemoreceptors

AKA aortic bodies and carotid bodies, receptors that are located in the aortic arch, subclavian arteries, and external carotid arteries and initiate chemoreflexes

Chemoreflex primary role

Adjust respiration to changes in blood chemistry

Chemoreflex secondary role

Vasoreflexes. Hypoxemia, hypercapnia, and acidosis stimulate chemoreceptors, acting through vasomotor center to cause widespread vasoconstriction, increasing BP, lung perfusion, and gas exchange

What are the two ways hormones can influence blood pressure?

• Vasoactive effects

• Regulating water balance

Angiotensin II

Potent vasoconstrictor that raises blood pressure by promoting sodium and water retention by the kidneys and increasing blood volume

Atrial Natriuretic Peptide (ANP)

Increases urinary sodium excretion (thus increasing water excretion), reducing blood volume and promoting vasodilation, ultimately lowering blood pressure

antidiuretic hormone (ADH)

Promotes water retention, raising BP. Also serves as a vasoconstrictor.

How does epinephrine and norepinephrine affect most blood vessels?

Bind to alpha-adrenergic receptors, promoting vasoconstriction

How does epinephrine and norepinephrine affect cardiac muscle blood vessels?

Binds to beta-adrenergic receptors, promoting vasodilation

What do arteries do in response to changing priorities?

Shift blood flow. During exercise, perfusion to the lungs, myocardium, and skeletal muscle increases, while perfusion to the kidneys and digestive tract decreases.

Venous return

The flow of blood back to the heart; relies on: pressure gradient, gravity, skeletal muscle pump, thoracic pump, and cardiac suction

Thoracic (respiratory) pump

During inhalation, the thoracic cavity expands and thoracic pressure decreases, while abdominal pressure increases, forcing blood upward

In what ways does exercise increase venous return?

• Heart beats faster and harder, increasing CO and BP

• Vessels of skeletal muscles, lungs, and heart dilate and increase flow

• Increased respiratory rate, increased action of thoracic pump

• Increased skeletal muscle pump

How does venous pooling occur?

Due to inactivity. Venous pressure is not enough to force blood upward, causing blood to pool in the lower extremities.

Circulatory shock

Any state in which cardiac output is insufficient to meet the body’s metabolic needs

Cardiogenic shock

Inadequate pumping of the heart, often caused by a MI

Low venous return (LVR)

shock due to cardiac output being low because too little blood is returning to the heart

What are two common cardiovascular disorders?

Coronary artery disease and heart murmurs

What often develops as a consequence of long-standing hypertension?

heart failure. hypertension causes increased afterload, which requires the ventricles to work harder to eject blood.